1. Field of the Invention
The present invention relates in general to a sealant dispenser, and more specifically, to a sealant dispenser for more precisely applying (or dispensing) a sealant onto a substrate, and a control method thereof.
2. Background of the Related Art
In general, liquid crystal display (LCD) is lighter and smaller by volume than a traditional image display device using cathode-ray tube (CRT). Therefore, making great strides in recent years, LCD has been widely applied in many display devices, such as computer monitors and TVs.
In case of the LCD, there is a certain space between an upper substrate and a lower substrate, and the space is filled with liquid crystal. In order for the LCD to perform its operation at optimum state, a cell gap of the lower and upper substrates should be maintained uniformly. In addition, in order to prevent excessive liquid crystal dispensing or insufficient filling of the space, it is very important to have a good positioning precision of a sealant coating for sealing the upper and lower substrates, and a proper control on the amount of coating being supplied to the substrates. This is because if the cell gap is not uniform or if liquid crystal is applied too much or insufficiently, the screen uniformity over the entire screen of an LCD will be deteriorated.
A sealant used to seal the space between the lower and the upper substrate not only maintains a uniform cell gap, but also seals the upper and the lower substrate.
A sealant is applied to a substrate by means of a sealant dispenser. The sealant dispenser includes a stage on which the substrate is mounted, and a head unit provided with a nozzle for discharging the sealant.
Here, the nozzle makes a relation motion to the substrate, and applies a sealant of a designated shape onto the substrate. In other words, the nozzle moves into X-axis and Y-axis directions relatively to the substrate, and applies the sealant onto the substrate. Also, the nozzle is movable into Z-axis direction to adjust its height with respect to the substrate.
FIG. 1 illustrates a head unit of a related art sealant dispenser.
As can be seen in the drawing, the head unit of the sealant dispenser includes a syringe 20 for storing a sealant, and a nozzle 32 coupled to a lower portion of the syringe 20, for discharging the sealant 50.
A long bar-shaped bracket 30 is connected to the syringe 20 in a horizontal direction, and the nozzle is installed on the end of the bracket 30. And, a distance sensor 40 for measuring a vertical distance between the nozzle 32 and the substrate 10 is installed in the vicinity of the nozzle 32.
More specifically, the distance sensor 40 is spaced apart a predetermined distance (L) from the syringe 20 which is filled with the sealant. Here, the syringe 20 and the distance sensor 40 must be installed in parallel because of their own volumes. Therefore, in order to put the nozzle communicated with the syringe as close as possible to the focal point of the distance sensor, the syringe and the nozzle must be installed in ‘L’ shape.
Usually, an optical laser distance sensor is used for the distance sensor 40. The bottom surface of the distance sensor has a “”, where a light emitting part 42 for emitting a laser beam 46 is installed on one side and a light receiving part 44 for receiving a laser beam 46 is on the other side.
Here, the light emitting part 42 emits a laser beam 46 towards the substrate, whereas the light receiving part 44 receives the laser beam reflected from the substrate. As such, the distance sensor 40 measures a distance between the substrate 10 and the nozzle 32.
When the laser beam path is changed due to a flexure on the sealant-applied surface of the substrate 10, the distance sensor measures the laser beam received to the light receiving part and transfers its measured value to a controller (not shown).
However, the related art sealant dispenser has the following problems.
Firstly, since the nozzle and the syringe are spaced a designated distance apart, the flow path of the sealant is bent in “L” shape. In detail, the distance sensor and the syringe of the related sealant dispenser are separately installed in the head unit while maintaining a designated distance from each other. This is why the flow path of the sealant has an “L” shape.
In that light, the related art dispenser requires a high pressure for discharging the sealant, and the usage of a highly viscous sealant was limited.
Secondly, according to the related art sealant dispenser, the nozzle is installed on the lower portion of the bracket, so that there is a great distance between the position on the substrate where the sealant is discharged and the position on the substrate where a laser beam is reflected. Therefore, the distance sensor cannot get an accurate a vertical distance from the point to which the sealant is actually discharged to the nozzle.
Thirdly, since the flow path of the sealant in the related art sealant dispenser is in “L” shape, the sealant is relatively slowly discharged. Hence, it is not clear exactly where sealant dispensing starts and ends.